2 fluid ounces light rum
1/2 fluid ounce hibiscus syrup
1/2 fluid ounce freshly squeezed lime juice
3 dashes aromatic bitters (such as Angostura®)
4 fluid ounces ginger beer, or more to taste
Combine rum, hibiscus syrup, lime juice, and bitters in a highball glass. Add enough ice to bring liquid volume up halfway; stir to combine. Top mixture with ginger beer.